This invention relates to oxidation-resistant nickel sulfide, cobalt sulfide, and mixtures thereof, and to a method for preparing such oxidation-resistant sulfides. More particularly, it pertains to oxidation-resistant sulfides on the type prepared by sulfiding ammonical liquors containing nickel and/or cobalt dissolved therein.
It is known in the art that nickel values can be recovered from reduced lateritic ores by leaching the ores with various types of aqueous ammoniacal solutions in the presence of an oxygen-containing gas, e.g., air. Such a process is commonly termed an "ammoniacal leaching process". In this process, certain metals normally associated with the nickel in the ores, e.g., cobalt and copper, are also extracted into the ammoniacal leach solution with the nickel. This is considered undesirable for a number of reasons, and processes have been devised to separate these unwanted components from the nickel-enriched leach liquor.
A common method of removing cobalt, copper, and sometimes various metallic impurities, from these leach liquors is by "selectively" precipitating the cobalt, copper and impurities as a mixture of sulfides, while leaving most of the nickel in solution. This is normally done by treating the pregnant leach liquor with controlled amounts of hydrogen sulfide or any of a variety of other sulfiding agents known to those skilled in the art. Some of the nickel in the leach liquor invariably precipitates with the cobalt, copper, and other impurities despite the selectivity of the precipitation. As a result, the resulting sulfide solids usually contain significant amounts of nickel and cobalt, with varying amounts of copper and impurities. The nickel to cobalt ratio in the sulfide precipitates can vary significantly depending upon such factors as the nickel to cobalt ratio of the ammoniacal solution and the amount of sulfiding agent used. The precipitated solids are usually removed from the main body of the nickel-enriched leach liquor in the form of a slurry, by successive gravity thickening and filtering, or by centrifuging.
I have found that prior to processing the separated slurry to recover the nickel, cobalt, and other metals contained in the solids fraction, it is often desirable to further concentrate the slurry by drying it in a manner that minimizes oxidation of the metal sulfides, e.g., in a non-oxidizing atmosphere. This is especially true if the sulfide material is to be processed in a different location from that where it is produced, and transportation of the material is therefore required. However, once the metal sulfides are dried at normal drying temperatures, e.g., about 300.degree.F. or below, I have also found that they tend to react readily with the oxygen in the air, sometimes almost pyrophorically. When this takes place, the dried sulfides react exothermically with the atmospheric oxygen to produce sulfates, which in turn then react with the water in the atmosphere to form hydrated compounds. Nickel sulfide, for example, may gain up to about 150% or more of its original weight after only a few days exposure due to this type of oxidation and hydration. For these reasons, the dried sulfide material can become difficult and hazardous to handle, transport or ship, thus requiring that special precautions and safety measures be taken. Furthermore, the exothermic reaction between the sulfide and oxygen results in a loss of potential energy that could be advantageously utilized during the subsequent refining of the sulfide for recovery of its metal values. The problem of rapid oxidation can occur with nickel sulfide alone or cobalt sulfide alone, as well as with mixtures of the two, and it is toward a solution of this problem that the present invention is directed.
It is, therefore, an object of this invention to provide nickel sulfide, cobalt sulfide, or mixtures thereof, in a form which is highly resistant to oxidation and thus can be conveniently and safely handled, transported and shipped without undesirable weight gains.
It is another object of this invention to provide a method for preparing such oxidation-resistant sulfides.
It is a further object of this invention to provide a method by which the rate of oxidation normally encountered with conventional nickel sulfide, cobalt sulfides or mixtures thereof can be substantially reduced.
It is a further object of the invention to provide a method for producing nickel sulfide, cobalt sulfide, or mixtures thereof which are substantially less hazardous and easier to handle, transport and ship than conventionally produced sulfides.
It is a still further object of this invention to provide a method for greatly minimizing undesirable weight gains of nickel sulfide, cobalt sulfide, or mixtures thereof during storage or shipment.
These and other objects of the invention will be apparent to those skilled in the art upon consideration of this entire specification and the accompanying drawings.